As a technique to simultaneously impart water repellency and oil repellency to a surface, it is known to treat an article with an organic solvent solution or aqueous dispersion of a polymer comprising polymerized units of a polymerizable monomer containing a polyfluoroalkyl group (a group having a structure wherein at least two and at most all of hydrogen atoms in an alkyl group are substituted by fluorine atoms, such a polyfluoroalkyl group will be hereinafter referred to as an “Rf group”) in its molecule, or a copolymer of such a monomer with another monomer.
Such water/oil repellency is attributable to formation of “a low surface energy surface” having a low critical surface tension on the surface due to a surface orientation of Rf groups on the coating film. It has been taken for granted that in order to attain both water repellency and oil repellency, orientation of Rf groups at the surface is important, and in order to realize the surface orientation of Rf groups, it is necessary to have constituting units derived from a monomer having a perfluoroalkyl group (a group having a structure wherein all of hydrogen atoms in an alkyl group are substituted by fluorine atoms, such a perfluoroalkyl group will be hereinafter referred to as an “RF group”) with at least 8 carbon atoms in the polymer.
However, recently, EPA (Environmental Protection Agency in U.S.A.) has pointed out that a compound having an RF group with at least 8 carbon atoms is likely to be decomposed in vivo and in the environment, and the decomposed product is likely to be accumulated, i.e. its environment load is high. Therefore, a copolymer for a water/oil repellent composition is required, which has structural units derived from a monomer having an RF group with at most 6 carbon atoms and containing no structural units derived from a monomer having an RF group with at least 8 carbon atoms.
However, in the case of a monomer having an Rf group with at most 6 carbon atoms, as compared with a monomer having an Rf group with at least 8 carbon atoms, the Rf orientation at the surface tends to be weak, and the water/oil repellency tends to be low. It is known to increase the water/oil repellency even in the case of a monomer having an Rf group with at most 6 carbon atoms, by copolymerizing it with a monomer not having an Rf group and having a high microcrystallite melting point (Patent Document 1), or copolymerizing it with a monomer having a crosslinkable functional group and not having an Rf group (Patent Document 2).
On the other hand, with a polymer composed solely of a monomer having an Rf group with at most 6 carbon atoms, it has been so far impossible to impart a sufficient water/oil repellency and excellent durability thereof.
Therefore, with respect to a monomer having an Rf group with at most 6 carbon atoms, particularly an RF group with at most 6 carbon atoms, a monomer and its polymer have been desired, whereby by polymerizing such a monomer, it is possible to obtain a polymer having a highly durable water/oil repellency.